1. Field of the Invention
This invention relates to linear light-emitting diode (LED) lamps and more particularly to a shock hazard-free linear LED lamp with a double safety mechanism.
2. Description of the Related Art
Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (no hazardous materials used), higher efficiency, smaller size, and much longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. As LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock become especially important and need to be well addressed.
In retrofit application of a linear LED (LL) lamp to replace an existing fluorescent tube, one must remove the starter or ballast because the LL lamp does not need a high voltage to ionize the gases inside the gas-filled fluorescent tube before sustaining continuous lighting. LL lamps operating at AC mains, such as 110, 220, or 277 VAC, have one construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LL lamps always fails a safety test, which measures through lamp leakage current. Because the line and the neutral of the AC main apply to both opposite ends of the tube when connected, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present risk of shock during re-lamping.
LEDs have a long operating life of 50,000 hours, much longer than conventional lighting devices do. One of the most important factors that detrimentally affect operating life of an LED-based lamp is high junction temperature of LEDs. While LEDs can operate 50,000 hours, the LED lamps do need a good thermal management in their heat sink design. A more efficient heat sink can effectively maintain LED junction temperature at a lower value and thus prolong the operating life of LEDs. Currently, the most cost-effective heat sink is made of metal. One of the drawbacks of using a metal as a heat sink in LL lamp application is electrical conductivity because shock hazard may occur when consumers touch the heat sink that is not well insulated from the LED printed circuit board (PCB) and the internal driver that powers the LEDs.
Today, such LL lamps are mostly used in a ceiling light fixture with a power switch on the wall. The ceiling light fixture could be an existing one used with fluorescent tubes but retrofitted for LL lamps or a specific LL lamp fixture. The drivers that provide a proper voltage and current to LEDs could be internal or external ones. Not like LL lamps with an external driver that is inherently electric-shock free if the driver meets the dielectric withstand standard used in the industry, LL lamps with an internal driver and a metallic heat sink present another shock hazard during relamping or maintenance, when a substantial leakage current flows from any one of AC voltage input through the metallic heat sink to the earth ground. Despite this disadvantage, LL lamps with an internal driver and a metallic heat sink still receive wide acceptance because they provide a long life, a stand-alone functionality, and an easy retrofit for an LL lamp fixture.
Any LL lamps will produce a small amount of leakage current through an internal electrical contact and the metallic heat sink because of the voltages applied and internal capacitance present in the LL lamp. When design flaws or material and workmanship defects appear, the electrical insulation in the LL lamp can break down, resulting in substantial leakage current flow. It mostly happens for small gaps between current-carrying conductors and the earth ground. When an LL lamp is operated under normal conditions, environmental factors such as dirt, contaminants, humidity, vibration, and mechanical shock can weaken the insulation and facilitate the current to flow through these small gaps and create a shock hazard to anyone who comes into contact with the metallic heat sink on the faulty LL lamps if care is not well taken.
As consumerism develops, consumer product safety becomes extremely important. Any products with electric shock hazards and risk of injuries or deaths are absolutely not acceptable for consumers. However, commercially available LL lamps with internal drivers and a metallic sink, which are used to replace fluorescent tubes, fail to provide a solution to these problems. In the present invention, a utility shock protection switch in addition to two end switches used on the lamp bases is adopted to fully protect consumers from possible electric shock injuries and deaths during relamping or maintenance.
Referring to FIG. 1 and FIG. 2, a conventional LL lamp 100 without shock protection switch comprises a metallic housing 110 with a length much greater than its radius, two end caps 120 and 130 each with a bi-pin 180 and 190 (not shown) on two opposite ends of the metallic housing 110, LED arrays 140 on an LED PCB 150, and an LED driver 160 used to generate a proper DC voltage from the energy supply of the AC main through internal wire connections 151 and 152 and provide a proper current to supply the LED arrays 140 through an internal wire connection 161 and 162 such that the LED's 170 on the PCB 150 can emit light. The PCB 150 is glued on a surface of metallic housing 110 by an adhesive with its normal parallel to the illumination direction. The bi-pins 180 and 190 on the two end caps 120 and 130 connect electrically to an AC main, either 110 V, 220 V, or 277 VAC through two electrical lamp sockets (not shown) located lengthways in an existing fluorescent tube fixture (not shown). The two lamp sockets in the fixture connect electrically to the line (L) and the neutral (N) wire of the AC main, respectively.
To replace a fluorescent tube with an LL lamp 100, one inserts the bi-pin 180 at one end of the LL lamp 100 into one of the two lamp sockets in the fixture and then inserts the bi-pin 190 at the other end of the LL lamp 100 into the other lamp socket in the fixture. When the line of the AC main applies to the bi-pin 180 through a lamp socket, there exists a shock hazard as long as the bi-pin 190 at the other end is not in the lamp socket because consumers who replace the linear LED lamp may touch the exposed bi-pin 190. The excessive current will flow from the bi-pin 180, an internal wire 151, driver 160, and an internal wire 152, and the bi-pin 190 to earth through his or her body—a shock hazard. This is most likely to happen in practice. To prevent consumers from injury for this shock hazard, Underwriters Laboratories (UL), uses its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do the current leakage test and to determine if LL lamps under test meet the consumer safety requirement.
On the other hand, when the line or neutral wire of the AC main connects to the bi-pin 180 through a lamp socket, no matter whether the bi-pin 190 at the other end is in the lamp socket or not, there exists another shock hazard because at this time, if a high voltage from a lighting strike, for example, applies to the AC main of the linear LED lamp, which happens to be a faulty one mentioned above, a high voltage breakdown, from the insulation-weakest point along an electrical path from the bi-pin 180, through internal wires 151, 161, and 162, the LED driver 160, and LED arrays 140 on the LED PCB, to the heat sink 110, will lead to an excessive leakage current flow to the heat sink 110. If the person who replaces the LL lamp 100 touches the heat sink 110, which also serves as the housing of the LL lamp, he or she will get electric shock because the current flows to earth through his or her body. This is likely to happen in practice. To prevent consumers from injury for this shock hazard, Underwriters Laboratories (UL), uses one of the procedures in UL 1993 Standards, Dielectric Voltage-Withstand Test, to determine if LL lamps under test meet the consumer safety requirements.